The generally accepted guideline for pack animal weight limits is that the load should not exceed 20 to 25 percent of the animal's body weight for extended trips over difficult terrain. This guideline has significant nuance depending on the animal's condition, the terrain, the duration of the trip, the weather, and the individual animal's fitness and load-bearing capacity.
A 1,000-pound mule at 20 to 25 percent capacity carries a load of 200 to 250 pounds, which must include the weight of the pack saddle, panniers, and all lashing materials — not just the contents of the panniers. The pack saddle itself may weigh 20 to 30 pounds, and a full set of panniers with hardware may add another 15 to 20 pounds before any gear is loaded, so the usable load capacity is always less than the theoretical limit.
Signs that an animal is carrying too much include a short choppy stride, sweating disproportionate to the effort, hollowing of the back, and reluctance to move forward. Any of these signs on trail should prompt an immediate stop to assess and reduce the load. Experienced packers sometimes load somewhat beyond the 25 percent guideline on short well-watered trips, but this requires experience and judgment that beginners should not attempt.